This invention relates to a test system for testing a semiconductor device, and in particular, to a test system suitable for testing a plurality of samples with one power supply at the same time.
Referring to FIG. 1, description will be made about a first related system for testing a semiconductor device (sample).
As illustrated in FIG. 1, a system for testing reliability of electromigration (EM) generally includes a sample 61 to be tested, a current source 62, and a voltmeter 63.
With such a structure, the current source 62 is provided for the sample 61. Current flows from the current source 62, and a voltage applied for the sample 61 is measured by the use of the voltmeter 63 on request.
In this example, although the number of the sample is set to one, a plurality of samples are generally arranged, and the current sources corresponding to the number are provided in the system. In this event, only one voltmeter is provided, and the respective voltages of the samples are sequentially measured by switching terminals.
Referring to FIG. 2, description will be made about a second related system for testing a semiconductor device (sample).
The system illustrated in FIG. 2 is generally used when the number of samples to be tested is increased.
In the system, samples 64.sub.1.about.64.sub.m (m.gtoreq.2) are connected for one voltage source (power supply) 65 in parallel. With this structure, a constant voltage is applied to both ends of each sample 64.sub.1.about.64.sub.m, and thereby, a current, which flows through the total of the samples, is measured by the use of an ammeter (galvanometer) 66 on request.
Under this circumstance, when either one of the samples 64.sub.1.about.64.sub.m is cut off, a current j, which flows through the ammeter 66, is reduced.
When the above-mentioned current-carrying test is continued, a relation between elapsed time and current is variable in the step-wise manner as represented in a graph illustrated in FIG. 3.
In this graph, the elapsed time until the current is changed in the step-wise manner from the current-carrying start time is defined as electromigration (EM) lifetime of the cut-off sample.
In the system illustrated in FIG. 1, the current sources corresponding to the sample number are required in the system when the constant current source is provided for each sample to be tested. In consequence, the sample number can not be set to a high value in this system.
On the other hand, in case that a plurality of samples are connected in parallel in the system illustrated in FIG. 2, the current is reduced in the step-wise manner when the sample is cut off. Thereby, the time until disconnection is judged as the lifetime of each sample.
However, resistance variation, which is not in a disconnection state, generally occurs in a lamination wiring pattern (for example, TiN/AlCu/TiN/Ti) which is used in the recent LSI (Large Scale Integrated Circuit). Consequently, the sample is also judged as a defective product by the resistance variation.
When the sample is variable in a resistance value, the current continues to flow through the sample, and a current value is changed in dependency upon the resistance value of the sample.
As a result, the current, which flows through the ammeter 66, is varied with time, and the constant current test can not practically carried out in this system.
In this case, it is difficult to judge the lifetime of each sample because the current value of the ammeter 66 is not changed in the step-wise manner.
From the above-mentioned reason, the system, in which the respective samples are connected in parallel, can not be used for the sample which has variable resistance value.
In general, when the EM reliability of the LSI wiring pattern is evaluated in the test, as the number of the sample is larger, lifetime estimation test can be more accurately performed in the LSI which is fabricated on the basis of the same specification.
In particular, initial faulty is often deviated from the log-normal plot. Therefore, unless the number of the sample is increased, the initial faulty can not be detected.